This invention relates to an antifogging device for a display glass that is placed in front of a displaying front plate of an indicating instrument.
Indicating instruments, as a rule, contain a more or less sealed-off air volume. Moisture that was able to penetrate easily results in a fogging of the display glass covering a displaying front plate on an indicating instrument. This is especially so in cases where the air volume, for example, because of Joule's heat loss of the components of the indicating instrument, is heated to a temperature exceeding the ambient air temperature and thus the temperature of the display glass.
An arrangement for removing fogging on display glasses of instruments is known on the basis of U.S. Pat. No. 2,697,242. In this case, the display glass is pivoted in the instrument housing. When the display glass is turned, it is guided over a wiping device that is stationarily connected with the housing.
This construction has the disadvantage that the removal of the fogging requires a manual operation that diverts the operating person. In addition, no measures are taken to prevent a repeated fogging.
It is also a disadvantage that by means of the wiping device, an unimpaired view of the display is prevented at least in areas.
In addition, it is known to heat instrument housings from the inside, for example, according to German Published Examined Patent Application (DE-AS) 23 20 021 or German Published Unexamined Patent Application (DE-OS) 21 31 919.
It is true that in the case of these known devices it is avoided that moisture that may have entered damages the metering works, especially in the case of temperatures below the freezing point. However, this does not prevent a fogging of the cooler parts of the instrument housing or of the display glass, but, on the contrary, causes it.
It is therefore an objective of the invention to provide an antifogging device for the display glass of an indicating instrument which can be built in a simple and cost-effective manner and by means of which a fogging of the display glass can be removed simply, reliably and automatically and a repeated fogging is prevented.
This objective is achieved according to the invention by an arrangement wherein the display glass is heated by means of the absorption of radiating heat emitted by the displaying front plate, which front plate is heated. In especially preferred embodiments the front plate is made of a heat conducting material and exhibits a high degree of heat emission on its surface facing the display glass. The front plate is preferably heated at its side opposite the display glass. In certain preferred embodiments the front plate is thermostatically heated by electric resistance heating PTC elements.
The advantages of the invention are that a fogging on a display glass of an indicating instrument is automatically removed by means of a device that can be built in a simple and cost-effective way and a repeated fogging is avoided.
Commercially available components can be used by applying so-called PTC-elements (i.e., resistors with a positive temperature coefficient) as an electric resistance heating, such as they are also used for the heating of outside mirrors on vehicles. In addition, because of the heating of the side of the display screen that faces away from the display glass, an adaptation of the indicating instruments is mostly superfluous since as a rule sufficient space is available there anyhow for such components. For this reason, the device is also suitable for retrofitting existing indicating instruments.
Further objects, features, and advantages of the present invention will become more apparent from the following description when taken with the accompanying drawings(s) which show, for purposes of illustration only, an embodiment in accordance with the present invention.